Fluvial-deltaic deposition within an evolving lacustrine basin, the Farson Sandstone Member of the Eocene Green River formation, southwestern Wyoming, USA.

摘要

Nine high-resolution detailed stratigraphic sections were measured in the Greater Green River Basin (GGRB) of southwestern Wyoming highlighting the lateral and vertical distribution of the Farson Sandstone Member and laterally equivalent Tipton Shale Member. Ten lithofacies were identified which define three facies associations; the profundal, littoral, and fluvial-deltaic associations. Stacking patterns of these facies and associations were used to constrain lateral correlations yielding five stratigraphic boundaries that bound four unique zones during the deposition of the Farson Sandstone and Tipton Shale members.;The stratigraphic zonation defined in this study correlates with previous studies which document changes in calcite and dolomite content based on X-ray diffraction, delta18O values of carbonates, and Fisher Assay oil yields from the ERDA White Mountain ;Spectral gamma ray measurements were used as a proxy for changes in mineralogy and suggest a change in provenance during deposition of Zones 3 and 4 as observed by an increase of Th and K concentrations. This may be related to previously documented uplift of the Wind River Mountains at this time. Lateral and vertical variations in U concentration are consistent with oil yield data and facies patterns that show a relative increase in organic matter preservation towards the basin center and during times of intermittently closed hydrology such as Zone 4.;Detailed stratigraphy of the Farson Sandstone Member and the laterally equivalent Tipton Shale Member show that topography on the basin floor influenced the distribution of lithofacies. Criteria for the existence of an intra-basinal high within the study are: 1) littoral deposits in Zone 1 on the proposed intra-basinal high correlate northward, towards the basin margin, to profundal mudstones, 2) the structural contour map of the Late Cretaceous Mesaverde Group defines an arch at the location of the proposed high suggesting it was a long-lived feature, and 3) a Bouguer gravity high correlates to this arch implying basement involvement. This intra-basinal high had a significant impact on the deposition of the Farson Sandstone Member within the basin during Zone 1, restricting fluvial-deltaic deposition in the north, west and northwest. At times it may have subdivided the basin into a northern and southern sub-basin. The location of the high correlates to the distribution of a series of enigmatic mounded facies within the Tipton Shale Member and overlying Wilkins Peak Member previously interpreted to relate to spring discharge. The arch in the underlying stratigraphy may have focused groundwater leading to discharge only the crest of the structure. Fractures along the crest may have future focused groundwater discharge. This relationship has not been documented before.